1. Field of the Invention
The present invention relates to a full charge capacity correction method for correcting a full charge capacity value of a rechargeable battery, and a full charge capacity correction circuit, a charging system, and a battery pack using the full charge capacity correction method.
2. Description of the Related Art
Conventionally, rechargeable batteries are widely used in a variety of devices and systems including electronic devices such as a mobile personal computer, a digital camera, a video camera and a mobile phone, vehicles such as an electric car and a hybrid car, hybrid elevators, power systems combining a photovoltaic cell or a power-generating device with a rechargeable battery, and battery-mounted devices and systems such as uninterruptible power source equipment.
In addition, from a usability perspective, electronic devices such as a mobile personal computer are adapted to display a remaining usable battery level that is charged in a rechargeable battery or to sound an alarm before the battery runs out.
Furthermore, in systems such as a photovoltaic generating system or a hybrid car, from a perspective of supplying power to a load in a stable manner, a rechargeable battery must be maintained in a state where the rechargeable battery is constantly charged to a certain level and, at the same time, generated surplus power and regenerative power must be absorbed. To this end, in order to prevent a fully-charged state in which a rechargeable battery can no longer be charged, charge control of the rechargeable battery is performed so that an SOC (state of charge) which represents a ratio (percentage) of a quantity of stored electricity that has already been charged to a full charge capacity (FCC) falls within a range of, for example, 20% to 80%.
In order to detect a remaining usable battery level that is charged in a rechargeable battery or calculate an SOC in this manner, a full charge capacity of the rechargeable battery must be determined. However, since a full charge capacity of a rechargeable battery decreases with deterioration of the rechargeable battery, using a factory-default full charge capacity without modification to calculate a remaining battery level or an SOC results in increased error.
In consideration thereof, a method of correcting or updating a full charge capacity is known in which a rechargeable battery is fully discharged even after the start of use by continuously discharging the rechargeable battery from a fully-charged state, whereby a full charge capacity is calculated by integrating discharge currents during the continuous discharge. However, with such a method, since a rechargeable battery that is in a used state in a device or a system must be fully discharged from a fully-charged state in order to correct the full charge capacity or the like, opportunities for correcting the full charge capacity become limited.
In consideration thereof, a method is known in which even if a rechargeable battery has not been fully discharged, a quantity of charged electricity from the start of charge until the rechargeable battery becomes fully charged is detected and the quantity of charged electricity is added to a quantity of stored electricity at start of charge (remaining capacity at start of charge) to calculate a full charge capacity (for example, refer to Japanese Patent Application Laid-open No. 2006-177764). In addition, Japanese Patent Application Laid-open No. 2006-177764 describes that the quantity of stored electricity at start of charge is determined based on battery voltage. Accordingly, a full charge capacity can be corrected even if the rechargeable battery is not fully discharged.
Meanwhile, according to the method described in Japanese Patent Application Laid-open No. 2006-177764, a data table representing a relationship between battery voltage and quantity of stored electricity (remaining capacity) is used when determining a quantity of stored electricity at start of charge based on battery voltage. In this case, as illustrated in FIG. 3 of Japanese Patent Application Laid-open No. 2006-177764, a terminal voltage of the battery has a correlation with an SOC that is a ratio (%) of a quantity of stored electricity (remaining capacity) to a full charge capacity.
Therefore, the quantity of stored electricity at start of charge must be calculated by first converting the terminal voltage of the battery into an SOC and then multiplying the full charge capacity with the SOC ratio. However, since the quantity of stored electricity at start of charge obtained in this manner is calculated based on a full charge capacity prior to correction, the quantity of stored electricity at start of charge contains an error of the full charge capacity.
As a result, there is a disadvantage in that a full charge capacity obtained by adding a quantity of charged electricity from start of charge until the rechargeable battery is fully charged to a quantity of stored electricity at start of charge (remaining capacity at start of charge) inevitably also contains errors.